When, on a substrate such as a printed-circuit board, another substrate or an electronic part is mounted, the electrodes of the electronic part or the like are soldered to the electrodes of the substrate. In this case, a soldering method by a reflow method is simple in which solder bumps are previously formed on the electrodes of the substrate and the electrodes of the electronic part or the like are soldered to the solder bumps.
Formation of solder bumps is most commonly performed by printing a solder paste with the use of a mask and then melting solder by heating. However, when such a printing method is used, the yield decreases due to the occurrence of bridging or variations in the amount of solder as the number of electrodes on a substrate increases so that the electrodes become miniaturized and the electrode pitch becomes narrower, which inevitably increases production costs.
Some methods for forming solder bumps with the use of a solder transfer sheet have recently been proposed. Patent Document 1 proposes a transfer sheet in which recesses formed in a solder resist layer are filled with solder powder. The pattern of the recesses of the transfer sheet is the same as that of the electrodes of a substrate.
When this solder transfer sheet is placed so that the recesses thereof are opposed to the electrodes of a substrate, and is then heated under pressure to melt the solder powder, solder bumps are formed on the electrodes of the substrate. It is essential for this method to perform the alignment of the transfer sheet. Further, since the transfer sheet needs to have recesses formed in a predetermined pattern, the production costs of the transfer sheet are increased.
On the other hand, as disclosed in Patent Document 2, there is a method using a transfer sheet in which one layer of solder powder is adhered to the entire surface of a supporting substrate using an adhesive layer. This transfer sheet is placed so that the solder powder surface thereof is opposed to the electrodes of a substrate. The substrate except the areas of electrodes is previously coated with a solder resist. Then, the substrate on which the transfer sheet has been placed is heated under pressure to melt the solder powder so that the electrodes are wetted with molten solder and the solder is adhered to the electrodes, but the solder resist-coated portion is not wetted with the molten solder.
After the molten solder is solidified by cooling, the transfer sheet is then removed from the substrate so that the substrate can be obtained in which the solder bumps are formed on the electrodes thereof. The solder in the portion of the transfer sheet opposed to the solder resist-coated portion is solidified while remaining adhered to the transfer sheet. This transfer sheet has solder powder adhered to the entire surface thereof but has no recesses, and therefore can be easily produced at low costs. Further, this transfer sheet makes it possible to eliminate the need for alignment, which is a troublesome operation, when the transfer sheet is placed on a substrate.
Further, as disclosed in Patent Document 3, there is also a method for forming solder bumps, as a method for forming solder bumps having a fine pattern, in which unlike the solder powder is melted during the transfer in the method disclosed in Patent Document 2, solder powder of a solder transfer sheet is transferred onto electrodes by solid-phase diffusion bonding caused by heating under pressure at a temperature lower than the melting point of solder.
Further, as described in Patent Document 4, there is additionally a method using a solder transfer sheet having a solder layer containing a solder powder sintered body obtained by sintering two or more layers of solder powder by solid-phase diffusion bonding, as a method for forming solder bumps by solder transfer, in order to perform the transfer, when forming relatively high solder bumps in a fine pattern without causing problems such as transfer defects including solder bridging and electrode destruction, which is similar to a case where one layer of solder powder is used.